Er - 16 Entity-Relationship Diagram ming 16.1 Semantic Data...

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139 16 Entity-Relationship Diagramming 16.1 Semantic Data Models Database design is fundamentally a task in data modelling. A data model is an architecture for data (chapter 10). Brodie has made a distinction between three generations of data model (Brodie, 1984): 1. Primitive Data Models . In this approach objects are represented by record- structures grouped in file-structures. The main operations available are read and write operations over records. 2. Classic Data Models . These are the hierarchical, network and relational data models. The hierarchical data model is an extension of the primitive data model discussed above. The network is an extension of the hierarchical approach. The relational data model is a fundamental departure from the hierarchical and network approaches. 3. Semantic Data Models . The main problem with classic data models like the relational data model is that they maintain a fundamental record-orientation. In other words, the meaning of the information in the database - its semantics - is not readily apparent from the database itself. The user of databases using the classic approach must consciously apply semantic information. For this reason, a number of so-called semantic data models have been proposed (King and Mcleod, 1985). Semantic data models (SDMs) attempt to provide a more expressive means of representing the meaning of information than is available in the classic models. Probably the most frequently cited of the SDMs is the Entity-Relationship data model (E-R model) (Chen, 1976). In the E-R model the ‘real world’ is represented in terms of entities, the relationships between entities and the attributes associated with entities. Entities represent objects of interest in the real world such as employees, departments and projects. Relationships represent named associations between entities. A department employs many employees.
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140 Information Systems Development An employee is assigned to a number of projects. Employs and is assigned to are both relationships in the Entity-Relationship approach. Attributes are properties of an entity or relationship. Name is an attribute of the employee entity. Duration of employment is an attribute of the employs relationship. 16.2 Entity Models An entity model is a model of the entities, relationships and attributes in some domain of discourse. An entity may be defined as ‘a thing which the enterprise recognises as being capable of an independent existence and which can be uniquely identified’ (Howe, 1986). A relationship can be defined as ‘an association between entities’ (Beynon-Davies, 1992). An attribute is a property of some entity. 16.2.1 Entities An entity may be a physical object such as a house or a car, an event such as a house sale or a car service, or a concept such as a transaction or order. Although the term entity is the one most commonly used following Chen (1976) we should really distinguish between an entity and an entity-type. An entity-type is
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This note was uploaded on 05/13/2010 for the course COMPRO CS401 taught by Professor Just during the Spring '10 term at Maharishi.

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Er - 16 Entity-Relationship Diagram ming 16.1 Semantic Data...

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